Tungsten alloy filaments for lamps and method of making

ABSTRACT

Tungsten filaments for lamps and methods of making the same in which a second phase material is dispersed in doped tungsten filament material to pair with bubbles produced by the dopant which are used to make the filament resistant to sag. The second phase material reduces migration of bubbles to hot spots on the operating filament and also retards bubble coalescence into groups. In a preferred embodiment of the invention, the second phase material additive is introduced in elemental form and the filament is heated in a nitrogen atmosphere to form a nitride of the additive.

[ Dec. 23, 1975 TUNGSTEN ALLOY FILAMENTS FOR LAMPS AND METHOD OF MAKING [75] Inventor: Ronald C. Koo, Weehawken, NJ.

[73] Assignee: Duro-Test Corporation, North Bergen, NJ.

[22] Filed: Jan. 31, 1973 [21] App]. No.: 328,246

Related US. Application Data [63] Continuation-in-part of Ser. No. 862,340, Sept. 30,

1969, abandoned.

[52] US. Cl. 29/1825; 75/205; 75/206; 75/207; 252/515 [51] Int. Cl. B22F 1/00; B22F 5/00 [58] Field of Search 75/205, 206, 207; 252/515; 29/182, 182.5

3,284,230 11/1966 Heytmijer et al. 75/207 3,300,285 l/l967 Pugh et al 75/207 3,351,438 l1/1967 Millner et al. 75/207 3,409,416 11/1968 Yates 75/205 Primary ExaminerBenjamin R. Padgett Assistant ExaminerB. H. Hunt Attorney, Agent, or Firm-Darby & Darby [57] ABSTRACT Tungsten filaments for lamps and methods of making the same in which a second phase material is dispersed in doped tungsten filament material to pair with bubbles produced by the dopant which are used to make the filament resistant to sag. The second phase material reduces migration of bubbles to hot spots on the operating filament and also retards bubble coalescence into groups. In a preferred embodiment of the invention, the second phase material additive is introduced in elemental form and the filament isheated in a nitrogen atmosphere to form a nitride of the additive.

20 Claims, 1 Drawing Figure US. Patent Dec. 23, 1975 3,927,989

INVENTOR. RONALD C. KOO

BY w hfiw ATTORNEYS TUNGSTEN ALLOY F ILAMENTS FOR LAMPS AND METHOD OF MAKING This application is a continuation-in-part of my copending application Ser. No. 862,340 filed Sept. 30, 1969, now abandoned Tungsten filaments for use in electric lamps, such as incandescent lamps, are well-known. Where incandescent lamps are of relatively high luminous efficiency, the tungsten filaments are in the form of coils, or coiled-coils, and are operated at temperatures in the order of 2,000C and above. To maintain the initial luminous efficiency of the lamp, that is the luminous efficiency of the lamp and particularly its filament which is present near the beginning of its operating time, over the lamp life requires that the filament be constructed to be resistant to sag. Such sag would distort the filament coil and result in an increase in the radiation heat loss thereby decreasing the luminous efficiency.

The relatively high stresses exerted upon the filament by its own weight at typical incandescent lamp operating temperatures demands that the filament material be exceptionally creep-resistant at elevated temperatures, particularly under vibration conditions to which lamps are often subjected. This requirement is not satisfied by ordinary pure tungsten because of its rapid decrease in mechanical strength with increasing operating temperature. To improve their high temperature strength, tungsten filaments are commonly doped during processing with a small, controlled amount of suitable material usually a metal composition including compounds of potassium, aluminum and silicon. The doping compounds are not limited to a combination of compounds of potassium, aluminum and silicon, since non-sag can also be achieved by doping with compounds of potassium and silicon only or with compounds of potassium, silicon and gallium the latter replacing aluminum. One typical compound used is an alkaline silicate and another is potassium aluminum silicate. In whatever combinations the metal containing non-sag dopant compounds may be, they always include an alkali metal such as lithium, sodium, potassium, rubidium or cesium, in order to provide non-sag properties to filaments for lamp applications. It has been recently discovered that it is the alkali metal retained in trace concentrations in the filaments that volatilizes at elevated temperature to form numerous submicroscopic bubbles. Doped tungsten filaments of this type have been in use for approximately years.

In addition to the non-sag properties required of the tungsten filament, it also should desirably possess long life at near the initial high luminous operating efficiency. Since the filament life depends primarily upon the efficiency (for the same coil design), an increase in life can only be accomplished at the expense of a de crease in luminous efficiency as long as the filament material remains unaltered.

Heretofore, other materials have been added to tungsten filaments for incandescent lamps for a variety of reasons. For example, in my US. Pat. No. 3,443,143, issued May 6, 1969 and entitled TungstemBase Alloy and Filament, pure rhenium or pure osmium is added to undoped, or pure, tungsten. Dispersed second phase material, in the form of particles of borides or nitrides of zirconium or hafnium, is also added to the mixture which is milled and then processed to form the filament wire. The net result is the production of a filament in which the glide of the partial dislocations of the tungsten constituent is retarded. This increases the mechanical strength of the filament at elevated temperatures but has relatively little effect on increasing filament life.

The present invention relates to improved tungsten filaments and more particularly to tungsten alloys and materials for dispersion in a doped tungsten filament, and the methods for preparation of the same, which are designed to improve the non-sag properties of a tungsten filament and especially to increase the filament life and/or the luminous efficiency of the filament. In accordance with the invention a tungsten filament, which is doped with a metal dopant material including at least one alkali metal for producing bubbles to improve the filaments sag resistance, is also provided with a second phase material which will pair with the bubbles in a strong bond. The bubbles which represent the vaporized alkali metal (e.g., potassium) comprise the third phase. The pairing reduces bubble migration to hot spots on an operating filament and also coalescence of bubbles into larger groups which form cavities in the filament material. Both of these effects, as explained in detail in the following specification, serve to lengthen the operating life of the filament. The physical princi ples of operation are different from those of my aforesaid patent. In the filament of the patent, there are no bubbles present for the second phase material particles to pair with since the tungsten is not doped.

The tungsten filament alloys of the instant invention are physically and chemically different in nature from prior art alloy systems. For example, tungsten containing dispersed particles of zirconium nitride basically is a two-phase alloy, i.e., a solid without any cavities. In the present invention, tungsten doped with the alkali metal containing dopant compounds and zirconium nitride is a three-phase system with the alkali metal (e.g., potassium) vapor within the bubbles representing the third phase at elevated temperatures (i.e., temperatures above the boiling point of the alkali metal in its pure form). Furthermore, this alloy is not intended to be a solid but to contain cavities, which are voids at room temperature due to the condensation of the alkali metal but become bubbles atelevated temperatures as the alkali metal within the bubbles vaporizes and develops high pressures. This behavior results from the insolubility of alkali metals in the tungsten lattice.

In contrast, a commercially produced non-sag tungsten filament doped with compounds containing an alkali metal is a single-phase material at room temperature prior to heating to elevated temperatures as defined previously. At elevated temperatures (above the boiling point of the pure alkali metal) as the bubbles are developed, the material can be considered as a two-phase system with the potassium vapor being the additional phase. The addition of, for example, nitride to the conventional doped tungsten thus converts it to a three-phase alloy to achieve trapping of bubbles.

Methods for preparing the filaments with the secondphase additive material are also disclosed in which: the material can be added in powdered form, using powder metallurgy techniques; dispersing the material into a slurry of the tungsten material; and adding the material to the tungsten to produce an alloy.

It is therefore an object of the present invention to provide improved tungsten filament materials for electric lamps.

A further object is to provide improved tungsten filament materials for electric lamps in which a second phase material is added to tungsten, to which already has been added a non-sag metal dopant material containing at least one alkali metal for producing bubbles, the second phase material combining with the bubbles to reduce their migration and coalescence characteristics.

An additional object is to provide methods of making improved tungsten filaments for lamps by the addition of a second phase material which will combine with bubbles produced in the filament to retard their migration and coalescence characteristics.

Other objects and advantages of the present invention will become more apparent upon reference to the following specification and annexed drawing in which the FIGURE shows a typical incandescent lamp made with the filament of the present invention.

The single FIGURE of the drawing shows in incandescent lamp having a light transmitting envelope 12. A base 14 is secured to the neck of the envelope which also has the usual conventional re-entrant stempress 16. A pair of lead-in wires 18 are sealed in the stem-press 16 and the ends (not shown) are in electrical contact with the respective terminals 19 and on the base. A filament 20 is mounted between the ends of the lead-ins 18 within the envelope. The filament 20 is of conventional shape, either coiled or coiled-coil, and is made in accordance with the invention as described below.

To aid in the understanding of the subject invention, a brief description of the prior art tungsten filaments using alkaline silicate dopants is presented.

In conventional processes for manufacturing tungsten filaments, a tungsten oxide slurry is doped with small, controlled amounts of potassium, aluminum and/or silicon compounds in the form of oxides or salts prior to the reduction of the tungsten oxide to metallic tungsten. These metal-containing dopant compounds always include at least'one alkali metal, i.e., lithium, sodium, cesium, rubium or potassium, to provide nonsag properties to the tungsten filament. Potassium is preferably employed as the alkali metal dopant constituent in this invention. In order to provide suitable nonsag characteristics, the tungsten filaments in an incandescent lamp should contain from about approximately 50 to about 5,000 parts per million by weight of the alkali metal constituent and preferably between about 100 and 4,000 parts per million by weight of the alkali metal constituent. The doped tungsten powder is then reduced by hydrogen, pressed into bars sintered in hydrogen at temperatures approaching 3,000C, swaged and drawn into wires. The wire is subsequently coiled into filaments, for example, by winding around a mandrel.

Heating the doped filament to the operating temperature of the incandescent lamp develops numerous internal bubbles of submicroscopic size, which are mostly in the range of 100 1,000A in diameter. The operating temperature of the incandescent lamp is generally above the boiling point of the alkali metal in its pure form, i.e.. above about 875C for potassium. These bubbles are formed as a result of the volatilization of the alkali metal dopant constituent which is insoluble in the tungsten matrix. The non-sag properties of a tungsten filament manufactured in this way are derived from the interaction of dislocations of the tungsten material with bubbles. The bubbles act as barriers blocking the movement of the material dislocations required for plastic deformation of the tungsten. Strengthening by bubbles is therefore similar to the wellknown strengthening by dispersion of solid, second phase particles. In the case of a tungsten filament, the magnitude of strengthening is a function of the mean inter-bubble spacing.

The alkali metal vapor bubbles which strengthen a tungsten filament also play a major role in determining the filament life. The bubbles produce relatively large cavities of various sizes. Their size often exceeds 10% of the wire diameter and the cavities are formed gradually in doped filaments during operation. The formation of these cavities is in a manner analogous to the thermal behavior of inert gas bubbles in nuclear materials, for example, the formation of helium bubbles in materials irradiated with alpha particles. The formation and growth of the large cavities in doped filaments result from the migration and coalescence of the submicroscopic bubbles containing the volatile alkali metal dopant. These cavities, which also frequently occur in clusters, reduce the effective cross-sectional area of the wire, resulting in an increase in the electrical resistance locally at the cavity location, and the formation of a hot spot (i.e., localized over-heating). This causes a filament failure by localized melting and breaking of the filament.

The sequence of events leading to filament burn-out is described below. As the filament is brought up to its operating temperature, the dopant volatilizes to form numerous submicroscopic bubbles (approaching bubbles/a which are aligned preferentially in rows parallel to the wire axis. Due to the rapid rate of diffusion at elevated temperatures, the bubbles so formed attain equilibrium size quickly, for example, within a few seconds at 2500K, and they can migrate readily by diffusion processes. In a uniform temperature zone, migration occurs at random walk, that is, Brownian movement, with very limited net movement. However, the existence of a temperature gradient, for example due to non-uniformity in pitch of the filament coil at or near the filament supports in a lamp in which the coil is mounted, provides a driving force such that the bubbles migrate preferentially in a direction up the temperature gradient and coalesce at the hot end of the filament. This further intensifies the temperature gradient, because bubble coalescence increases the total bubble volume due to the balance between the internal pressure P and the surface tension 2y/R, where 'y is the surface free energy of tungsten and R is the equilibrium radius of the bubble. Bubble coalescence decreases the effective cross section of the wire at the point of coalescence and this intensifies the temperature gradient. The increase in temperature gradient in turn increases the driving force for unidirectional migration of bubbles, forming a cycle that continues until local overheating causes melting of the filament and failure.

The life of a doped filament is primarily determined by the kinetics of bubble coalescence ending in filament failure (burn-out). Evaporation of tungsten accelerates the process by reducing the external diameter of the wire at the hot spot, further reducing the cross-sectional area and increasing the temperature. Contrary to the expectation, from the standpoint of evaporation, that burn-out should occur at locations of maximum initial temperature. filament failure occurs predomi nantly where T(dt/d.r) is a maximum. where dt/dx is the temperature gradient providing the driving force and T is the temperature determining thekinetics of bubble migration. Coiled and coiled-coil filaments generally have a much longer life than uncoiled filaments oper ated at the same temperature because any temperature gradient due to material inhomogeneity is minimized by the radiation effect of an adjacent portion of the filament coil. It is thermodynamically impossible to suppress the kinetics of the bubble migration and coalescence entirely without elimination of the bubbles, which are required to make the filament more sagresistant. However, a substantial increase in filament life can be achieved through a retardation of the kinetics of bubble migration and coalescence, while still retaining the advantage of the bubbles for reducing sag.

According to the invention, several non-sag filament compositions which reduce bubble migration and the methods of making these are disclosed. In general, the compositions include the addition to the doped tungsten matrix discussed above of a dispersion of submicroscopic, second phase particles. The second phase materials are insoluble in the tungsten matrix or the metal vapor within the bubbles. The compositions so formed operate so that a migrating bubble, upon collision with a particle, becomes attached to it forming a bubble-particle pair, which is immobile. This attachment reaction is thermodynamically favored since the pairing of a bubble with a particle leads to a reduction in free energy that results from a decrease in the interfacial energy exceeding the reduction in the entropy of mixing. This is analogous to the addition of glass beads to a container of water having air bubbles, which tend to adhere to the glass beads as a result of the reduction of the total interfacial surface free energies. Similarly, the bubbles, in doped tungsten, which are filled with potassium vapor under high pressures at elevated temperatures (i.e., filament operating temperatures) will also tend to adhere to the second-phase particles (e.g., zirconium nitride) which are immiscible with the tungsten matrix and the potassium vapor. The bubble particle pairing phenomena does not involve a phase change or a chemical reaction.

With a sufficient concentration of second phase particles, the number of mobile bubbles per unit volume available for participation in the coalescence process is greatly reduced. The presence of the dispersed second phase particles also has the additional advantage of improving the sag resistance properties of the filament since it provides additional barriers to dislocation movements.

The thermal stability of a bubble-particle pair de creases in the presence of a temperature gradient. A bubble can free itself from the particle if the driving force due to the temperature gradient is large enough to overcome the binding energy of the bubble-particle pair. However, in view of the large binding energy, particularly in the immediate vicinity of an intense hot spot, a very steep gradient is required to break up a pair. It should be noted that the prevention or retardation of the formation of a hot spot at its embryonic stage is a major factor which has to be satisfied to significantly increase the filament life.

The dispersed second phase material used for achiev ing the retardation effect on bothbubble migration and coalescence should preferably satisfy the following criteria. The material must be thermally stable at the operating temperature of the filamentLThis requirement would make necessary the useof compounds which preferably do not decompose below at least 1,800C. Furthermore, the compounds must be chemically stable in the tungsten matrix of the tungsten and the dopant. Carbides and borides, in spite of their high melting point, are undesirable because the formation of tungsten carbide and boride will not only cause instability of the dispersed phase but will also embrittle tungsten.

In accordance with the invention, the most suitable additions for the second phase material are oxides, nitrides and nitrates of the high melting-point elements, in particular, those of the transition elements such as titanium, zirconium, hafnium, tantalum, columbium, ruthenium and thorium, or oxides and nitrides of the rare earth elements such as disprosium, erbium, europium, gadolinium, holmium, lanthanum, neodymium, samarium, ytterbium and yttrium as well as oxides and nitrides of strontium. Additive materials believed to be particularly suitable are compounds of zirconium, hafnium, or tantalum nitride in the range of A to 1 /2 vol.% of the total composition when incorporated into the standard alkali-silicate doped tungsten matrix. Either a single one or a combination of the additive compounds can be used.

The second phase compound material should be finely dispersed and be of submicron size. There are several methods by which a fine dispersion can be incorporated into the tungsten matrix by powder metallurgy techniques. These are explained below.

METHOD 1 Considering the process for making the standard tungsten matrix described previously, the chosen additive compound, or combinations of compounds, is prepared in submicron size and is introduced into the doped tungsten powder prior to consolidation by pressing and sintering. The additive also can be introduced into the tungsten oxide slurry prior to the hydrogen reduction. The latter, however, can only be carried out for compounds stable in hydrogen up to 900C through the reduction process. Method 1 limits the particle size of the dispersed second phase material in the finished wire to the initial particle size of the addition introduced into the powder or slurry before consolidation.

METHOD II To adhieve a finer dispersion of the particles of second phase material, the additive compound, or compounds, are selected to be a metal nitrate or nitrite and are introduced into a tungsten oxide slurry. Upon drying under heat, the metal nitrate or nitrite additive is converted into oxides. Because of the porosities in the base tungsten oxide matrix, the resulting particles of the oxide of the material added are finely dispersed within the doped tungsten oxide crystals and remain as fine particles throughout processing down to fine wire.

METHOD Ill Another method of creating a fine dispersion, which is the preferred embodiment in accordance with the invention, is to add the second phase additive element in a metallic state, rather than as a compound, and form a solid solution with the doped tungsten, The alloy so formed, after being drawn to fine wire or coiled into filaments, is then heat treated in a controlled atmosphere to precipitate a dispersed second-phase from solid solution. For example, a dispersion of hafnium nitride can be obtained by blending hafnium powder with the doped tungsten powder prior to consolidation.

The finished wire or coiled filament is then internally nitrided, i.e., heating it at temperatures above 1,000C in a nitrogen-bearing atmosphere such as pure nitrogen or ammonia. There are two distinct advantages to this method. First, since the addition remains in solid solution during fabrication, the alloy is much more ductile than alloys containing a dispersed second-phase, so that material shrinkage during fabrication is significantly reduced. Another advantage is that the size, morphology and the distribution of the nitride can be controlled by the heat treatment. With decreasing temperature of nitriding, the particles of the nitride compounds formed are smaller in size and are distributed prefentially along dislocations. This has the advantage of pinning the dislocation substructure generated by plastic deformation with a resulting increase in the recrystallization temperature by a few hundred degrees Centigrade. Since doped tungsten wire normally recrystallizes at 2,000 2,300C, the recrystallization temperature of nitrided doped filaments can easily exceed 2,500C. The retention of the cold-worked substructure by the dispersed phase further increases the sag resistance of filaments operated at temperatures below 2,500C.

METHOD IV Another method to create a fine dispersion of nitrides of zirconium, hafnium, titanium, and/or tantalum in doped tungsten filaments, which is another suitable embodiment in accordance with the present invention, is to add the nitride to the doped tungsten powder by decomposing the corresponding metal halide in the presence of nitrogen prior to the consolidation of the tungsten powder by pressing and sintering. By heating the doped tungsten powder to temperatures above l,OOOC in an atmosphere containing a halide vapor such as zirconium iodide or zirconium chloride and nitrogen, the halide decomposes and deposits metallic zirconium on the tungsten powder. The zirconium, as it deposits 'on the tungsten powder, is simultaneously converted to zirconium nitride in the presence of nitrogen. This results in a very fine dispersion of zirconium nitride on the tungsten powder. The size and amount of zirconium nitride particles can be readily controlled by the condition under which the zirconium halide decomposes, e.g., the temperature and partial pressure of the halide can be adjusted accordingly. This method can also be applied to the doped (blue) tungsten oxide as well as the reduced metallic powder.

WORKING EXAMPLE NO. 1

A non-sag tungsten filament containing a dispersed second phase material in accordance with this invention was prepared by incorporating the nitride onto the doped tungsten powder prior to consolidation. One kilogram of potassium aluminum silicate doped tungsten powder (commercially available as Sylvania NS (Non-Sag) Tungsten Filament Powder) was fed slowly into a reaction chamber through which a continuous flow of zirconium tetrachloride, nitrogen and hydrogen gases heated to 1,300C, was maintained. The total pressure of the gas mixture in the chamber was one atmosphere with ratios of partial pressures of zirconium tetrachloride, nitrogen, and hydrogen being 1:2:1, respectively. The reaction chamber was made of a vertically positioned spiral reaction tube with the tungsten powder entering into the chamber at the top and the gas mixture flowing into the chamber at several spaced apart locations. The tungsten powder slowly moved downward in the spiral chamber with the aid of a mechanical vibrator. The powder was allowed to remain in the reaction chamber for one minute, and then was placed in a hydrogen atmosphere cooling chamber and allowed to cool to room temperature (about F). The cooled material was allowed to settle into a ceramic container.

Since a sufficient amount of atomic nitrogen was present within the reaction chamber as the zirconium metal was deposited on the tungsten powder, it reacted simultaneously with nitrogen thus forming zirconium nitride particles.

The materials and condition described herein provided a deposition rate on the substrate of about 250 microns per hour, and ,zirconium nitride particles in a size range of about 0.1 microns to about 1.0 microns were readily obtained in 1 minute of reaction time. The resulting tungsten powder, containing very fine particles of zirconium nitride was then pressed, sintered, and consolidated into non-sag lamp filaments using conventional powder metallurgical techniques.

WORKING EXAMPLE NO. 2

The same materials and conditions were used as in Example 1. However, the reaction vessel temperature was maintained at 1,100C. The resulting zirconium nitride particles deposited on the tungsten powder were on the order of less than 0.1 microns. The resulting tungsten powder was pressed, sintered and consolidated into non-sag lamp filaments using conventional powder metallurgical techniques.

The procedures outlined in Examples 1 and 2 are not restricted to metallic tungsten powder and can also be carried out on doped tungsten (blue) oxide prior to its reduction in metallic tungsten by hydrogen. Similarly, the methods of Examples 1 and 2 are not restricted to the decomposition of a chloride and will work equally well for any halide, e.g., iodides of zirconium, hafnium or titanium can also be used successfully. However, chlorides are generally preferred because of their higher vapor pressure and lower cost.

Method III is more useful in internal nitriding than for internal oxidation, that is, the formation of oxide compounds with the original additive alloying element by heat treating the solid solution alloy in an oxygenbearing atmosphere. Because of the thermal instability of tungsten nitride at elevated temperatures, the formation of the nitride of the additive alloying element occurs without forming a case, or envelope, of tungsten nitride. For internal oxidation, the formation of a case of tungsten oxide reduces or prevents the reaction with the alloying second-phase element. Tungsten oxide is more easily formed than the oxide of the additive element. It is particularly unfavorable if the free energy of formation of the internal oxide of the second phase material is larger than that for the formation of tungsten oxide.

In the methods described above, the most often used range of the concentration of the dispersed second phase material required has been found to be /2 vol.% to 1 vol.%. The optimum amount of addi ive depends upon the particle size and the degree of dispersion. For very fine particles of the additive, for example, 0.1; in diameter, 0.1 vol.% is sufficient to reduce the kinetics of bubble coalescence. For coarse particles distributed non-randomly, a concentration as high as 5 vol.% is more in order.

Since the addition of a dispersed, second phase material generally reduces the ductility of tungsten, it may be beneficial to incorporate an additional alloying element to promote ductility. This can be achieved by the addition of 3 wt% wt% of rhenium or osmium in solid solution. Such an addition is particularly effective in improving the cold-shock resistance of the filament.

Although any alkali metal will-serve as the bubble producing constituent of the dopant compound in the invention, potassium is preferably employed. The alkali metal non-sag dopant can be introduced into the tungsten during processing in the form of a halide (e.g.,

potassium chloride), a nitrate (e.g., potassium nitrate) or a silicate (e.g., potassium silicate).

The various compositions to reduce bubble coalescence set forth above are based upon the formation of pairs of clusters of additive particles with bubbles. This occurs for particles of the additive which do not react chemically with the vapor of the dopant originally used to fonn the bubbles. A similar effect is also obtained if the bubble forming dopant forms a solid solution with the dispersed phase additive material, since such a reaction simply absorbs the bubble and thus reduces the number of bubbles participating in the coalescence process. Since only a fraction of bubbles contribute to hot spot formation, the absorption of bubbles by the dispersoids does not reduce the sag resistance, especially in the presence of a stable dispersed phase.

The amount of increase of the life of a doped tungsten filament by the compositions of the present invention varies for different lamp types, since other pro cesses in addition to bubble coalescence also operate in doped tungsten filaments. The present invention is most effective for filaments operating under the condition that bubble coalescence plays the dominant role. This becomes the controlling mechanism in filaments operating at high temperatures, that is, above 2,000C with a relatively large diameter, that is, above 1 mil. With decreasing operating temperature and wire diameter, the contribution to hot spot formation by other processes becomes increasingly significant. These processes include thermal grooving, facet formation, and electrotransport phenomena.

What is claimed is:

1. In a tungsten filament of the type containing a filament dopant compound including at least one alkali metal which produces a second phase consisting of bubbles at filament operating temperatures to improve the sag resistant properties of the filament, the improvement comprising a third phase material selected from the group consisting of titanium, zirconium, hafnium, tantalum, ruthenium, strontium and columbium added to the filament for combining with the bubbles to reduce their migration and coalescence.

2. A filament of the type as in claim 1, wherein said third phase material is an oxide compound of one of the group of materials named.

3. A filament of the type as in claim 1, wherein said third phase material is a nitride compound of one of the group of materials named.

4. A filament of the type as set forth in claim 1, wherein said third phase material comprises from about 4% to about 5% of the total volume of the filament.

5.. A filament as in clain 1, wherein the alkali metal is potassium.

6. a filament as in claim 1, wherein said alkali metal is sodium.

7. A filament as in claim 1, wherein said alkali metal is lithium.

8. In a tungsten filament of the type containing a filament dopant compound including at least one alkali metal which produces a second phase consisting of bubbles at filament operating temperatures to improve the sag resistant properties of the filament, the improvement comprising a third phase material selected from the group consisting of disprosium, erbium, eropium, gadolinium, holmium, lanthanum, neodymium, Samarium, ytterbium and yttrium added to the filament for combining with the bubbles to reduce their migration and coalescence.

9. A filament of the type as in claim 8, wherein said third phase material is an oxide compound of one of the group of materials named.

10. A filament of the type as in claim 8, wherein said third phase material is a nitride compound of one of the group of materials named.

11. A filament of the type as in claim 1 wherein said third phase material is selected from the group consisting of the nitride compounds of zirconium, hafnium and tantalum.

12. A filament of the type as in claim 1 1, wherein said compound comprises from about to about l/2% of the total volume of the filament.

13. The method of making a tungsten filament comprising the steps of adding to tungsten a dopant material including at least one alkali metal for producing a second phase consisting of bubbles upon operation of the filament, adding a third phase material selected from the group consisting of titanium, zirconium, hafnium, tantalum, ruthenium and columbium for combining with the bubbles, forming a composition of tungsten dopant and third phase material into a solid mass, and drawing a filament wire from said solid mass.

14. The method as in claim 13, wherein the tungsten is supplied in powdered form and the third phase material is also supplied in powdered form.

15. The method as in claim 14, wherein the tungsten is formed into a slurry of tungsten oxide and the third phase material is introduced in powdered form into the slurry.

16. The method of claim 15, wherein the third phase material is a compound selected from the group consisting of metal nitrates or nitrites and is introduced into the slurry, and further comprising the step of heating the slurry with the compound to convertthe compount to its oxide form.

17. The method of claim 13, wherein the third phase material is added to tungsten in elemental form, and further comprising the step of forming an alloy of the tungsten and the third phase material.

18. The method of claim 17, wherein the alloy is heated in a nitrogen atmosphere to internally nitride the third phase material.

19. A filament as in claim 5, wherein said dopant is potassium aluminum silicate.

20. A filament as in claim 5, containing from about to about 5,000 parts per million by weight of potassium. 

1. IN A TUNGSTEN FILAMENT OF THE TYPE CONTAINING A FILAMENT DOPANT COMPOUND INCLUDING AT LEAST ONE ALKALI METAL WHICH PRODUCES A SECOND PHASE CONSISTING OF BUBBLES AT FILAMENT OPERATING TEMPERATURES TO IMPROVE THE SAG RESISTANT PROPERTIES OF THE FILAMENT, THE IMPROVEMENT COMPRISING A THIRD PHASE MATERIAL SELECTED FROM THE GROUP CONSISTING OF TITANIUM, ZIRCONIUM, HAFNIUM, TANTALUM, RUTHENIUM, STRONTIUM AND COLUMBIUM ADDED TO THE FILMENT FOR COMBINING WITH THE BUBBLES TO REDUCE THEIR MIGRATION AND COALESCENCE.
 2. A filament of the type as in claim 1, wherein said third phase material is an oxide compound of one of the group of materials named.
 3. A filament of the type as in claim 1, wherein said third phase material is a nitride compound of one of the group of materials named.
 4. A filament of the type as set forth in claim 1, wherein said third phase material comprises from about 1/4 % to about 5% of the total volume of the filament. 5.. A filament as in clain 1, wherein the alkali metal is potassium.
 6. a filament as in claim 1, wherein said alkali metal is sodium.
 7. A filament as in claim 1, wherein said alkali metal is lithium.
 8. In a tungsten filament of the type containing a filament dopant compound including at least one alkali metal which produces a second phase consisting of bubbles at filament operating temperatures to improve the sag resistant properties of the filament, the improvement comprising a third phase material selected from the group consisting of disprosium, erbium, eropium, gadolinium, holmium, lanthanum, neodymium, samarium, ytterbium and yttrium added to the filament for combining with the bubbles to reduce their migration and coalescence.
 9. A filament of the type as in claim 8, wherein said third phase material is an oxide compound of one of the group of materials named.
 10. A filament of the type as in claim 8, wherein said third phase material is a nitride compound of one of the group of materials named.
 11. A filament of the type as in claim 1 wherein said third phase material is selected from the group consisting of the nitride compounds of zirconium, hafnium and tantalum.
 12. A filament of the type as in claim 11, wherein said compound comprises from about 1/4 % to about 1 1/2 % of the total volume of the filament.
 13. The method of making a tungsten filament comprising the steps of adding to tungsten a dopant material including at least one alkali metal for producing a second phase consisting of bubbles upon operation of the filament, adding a third phase material selected from the group consisting of titanium, zirconium, hafnium, tantalum, ruthenium and columbium for combining with the bubbles, forming a composition of tungsten dopant and third phase material into a solid mass, and drawing a filament wire from said solid mass.
 14. The method as in claim 13, wherein the tungsten is supplied in powdered form and the third phase material is also supplied in powdered form.
 15. The method as in claim 14, wherein the tungsten is formed into a slurry of tungsten oxide and the third phase material is introduced in powdered form into the slurry.
 16. The method of claim 15, wherein the third phase material is a compound selected from the group consisting of metal nitrates or nitrites and is introduced into the slurry, and further comprising the step of heating the slurry with the compound to convert the compount to its oxide form.
 17. The method of claim 13, wherein the third phase material is added to tungsten in elemental form, and further comprising the step of forming an alloy of the tungsten and the third phase material.
 18. The method of claim 17, wherein the alloy is heated in a nitrogen atmosphere to internally nitride the third phase material.
 19. A filament as in claim 5, wherein said dopant is potassium aluminum silicate.
 20. A filament as in claim 5, containing from about 100 to about 5,000 parts per million by weight of potassium. 